1. Field of the Invention
This invention relates to focus apparatus for use in optical systems, and more particularly, to such apparatus for use as a focus error detector in optical systems.
2. Description of Related Art
In optical systems such as optical information storage systems, it is necessary to maintain the scanning optical beam in sharp focus on the recording element and to also precisely control the beam's lateral position to assure that a desired data track is being followed. In other optical systems, such as drum printers with movable optical heads, it is necessary to control the focus position of the print head in relation to the drum in order to achieve controllable print densities. A variety of detection systems are known for deriving a focus error signal such as systems using knife edge detection, astigmatic beam detection, and spot size detection. None of these systems is entirely satisfactory. Certain of the systems are sensitive to positioning errors of the beam on the detector. Other known systems are sensitive to aberrations induced in the focus beam by the focusing when the beam is not well positioned.